1. Technical Field
The present invention relates to the technical field of industrial wireless sensor networks, and in particular to a clock synchronization frequency offset estimation method adapted to a WIA-PA network.
2. Description of Related Art
An industrial wireless sensor network features low power consumption and low cost, and has become a hotspot as to the development direction in the industrial control field. The WIA-PA (Wireless Networks for Industrial Automation Process Automation) standard is based on the IEEE 802.15.4 standard for short-range wireless communications, and the data link layer of the WIA-PA is compatible with and expands a superframe structure in IEEE 802.15.4. The data link layer of the WIA-PA supports a time slot based frequency hopping, retransmission, CSMA and TDMA mixed channel access mechanisms, thereby guaranteeing the reliability and real-time capability for the transmission. Therefore, to guarantee the reliability of the TDMA mechanism, it is extremely important to perform time synchronization on the devices in the WIA-PA network.
According to different recording manners of the time information in a time synchronization process, the time synchronization methods for the wireless sensor networks can be divided into the following three classes: a one-way based synchronization algorithm, a two-way message exchange based synchronization algorithm, and a receiver-receiver based synchronization algorithm. The one-way based synchronization algorithm mainly includes an FTSP algorithm and a DMTS algorithm, the two-way message exchange based algorithm mainly includes an LTS algorithm and a TPSN algorithm, and the receiver-receiver based synchronization algorithm mainly refers to an RBS algorithm. The time synchronization mechanism of the WIA-PA network mainly involves beacon frame synchronization and command frame synchronization, where a child device receives a beacon frame attached with time information, which is broadcast periodically by a time source, records a local time when the beacon frame is received, and simply compares the two times to obtain a clock offset between the present device clock and a standard clock, thereby adjusting the local clock with the clock offset; and moreover, secondary synchronization can also be performed with a dedicated time synchronization command frame of the WIA-PA network.
In the WIA-PA network, the transmission of a message is necessary to complete the time synchronization between nodes, however, various delays in message transceiving have an important effect on the precision of the time synchronization. FIG. 1 provides an exploded diagram of a time delay in a message transceiving process. As shown in FIG. 1, to better analyze an error in the message transmission, the time delay for message transceiving can be divided into six parts, i.e. send time, access time, transmission time, propagation time, reception time and receive time, which are key factors affecting the time synchronization.
At present, the time synchronization mechanism of the WIA-PA network has the following major problems: first, the clock offset is simply estimated only in a current beacon frame synchronization mechanism without estimating and compensating the more important frequency offset, resulting in that time errors rapidly accumulate in a time interval of the synchronization period, and the time source has to increase the broadcast frequency of the beacon frame to maintain the basic time synchronization precision of the network; second, in a time parameter estimation process, the effects of the six types of delays, including send time, access time, transmission time, propagation time, reception time and receive time, on an estimation result in the message transceiving process not being excluded, resulting in a larger deviation between the adjusted time and the standard time, and the reduction of synchronization precision; and finally, a general clock offset and frequency offset joint estimation method has the defects of high computation burden and higher complexity, and the situation, which is more conforming to an actual scene, that the child node compensates the local time in each synchronization period and then separately estimates the relative frequency offset as provided by the present invention is rarely taken into consideration.